The National Aquarium in Baltimore and Constellation have agreed to a 25-year electricity supply deal that includes the construction of a solar power plant to supply nearly half of the Aquarium’s electricity. As part of the agreement, announced by the two organizations on Thursday, Constellation will fund, build and operate a 4.3-megawatt, grid-connected solar project in Cambridge, developed with OneEnergy Renewables. The plant, scheduled to be ready in March, is designed to generate an estimated 5.8 million kilowatt hours during its first year, more than 40 percent of the Aquarium’s annual electricity needs. “This agreement with Constellation allows us to ‘walk our talk’ as a conservation organization,” said John Racanelli, CEO of the National Aquarium. “Now, a significant percent of our electricity will come from clean, renewable energy. We hope this will help set a precedent for supporting large-scale, off-site projects and inspire others to take action.” The Cambridge project, comprised of approximately 14,500 photovoltaic panels, will cover about 22 acres.